DES MOINES, NOV. 7 -- Sen. Albert Gore Jr. (D-Tenn.) came here tonight hoping to shake up the Democratic presidential race with a wholesale attack on Iowa's role in the presidential nominating system, but instead found himself making news on a far different subject -- his use of marijuana as a young man.

"I assure you it wasn't a strategic decision," Gore said in the only light moment in a 30-minute news conference before the biggest Democratic political event of the year, the annual Jefferson-Jackson Day Dinner.

Gore, the darkest horse in the Iowa field, had hoped to set himself apart from his five rivals by attacking the heavy investments of time that Iowans demand -- and receive -- from presidential contenders.

The test of a candidate, he said, "is not how many times you've met us, which of us got here first, came here most, spent the most money, or bought the most tickets to this dinner. The test of a candidate is what he will do for the nation."

Gore and former Arizona governor Bruce Babbitt today acknowledged that they had used marijuana as young men. The drug issue distracted attention from what was to be a day of parades, speeches and hoopla as about 7,000 Democrats gathered here.

"It's another small pothole on the path to the presidency. Instead of a huge wonderful hurrah for our candidates, we're going to be hearing a lot about who did what 20 years ago," said Lynn Cutler, vice chairman of the Democratic National Committee.

"I've had it with this whole thing -- picking at people's personal lives, about things that aren't relevant," Cutler added.

Today was one of the busiest political days in Iowa history. More presidential candidates -- 10 -- were in the state than at any time in memory. Among the candidates, only Republicans Marion G. (Pat) Robertson and Alexander M. Haig Jr. did not spend time in the state.

The annual Jefferson-Jackson Dinner is the largest single Democratic event of the year here, and the six Democratic campaigns treated it as a warmup for the state's Feb. 8 precinct caucuses.

The campaigns considered it a major organizational test, and each has been working for weeks to make sure it could muster a respectable following.

For the candidates, the dinner provided one of the biggest forums of the year to test their messages.

Gore, running last in polls here, attacked Iowa as a political testing ground and hit the litmus-test issues liberal-oriented interest groups use to grade candidates.

He charged that the Iowa caucuses "reward ideological purity rather than intellectual honesty" and suggested that some candidates "seem to think you care more about your own interest than about the national interest."

He singled out a liberal peace group called STARPAC on the litimus-test issue. "If it takes a passing grade on every one of STARPAC's tests, a candidate like John Kennedy or Harry Truman or Franklin Roosevelt could never win the Iowa caucuses.

"There is something wrong with a nominating process that gives one state the loudest voice and then produced candidates who cannot even carry that state" in general elections, Gore said. "Iowa is a competitive state. Yet we have not been able to win it {in a presidential race} for the past 23 years.

"We have lost four of the last five elections . . . . Isn't it time for a change?" he added.

Sen. Paul Simon (D-Ill.) used the opportunity to picture himself as an old-fashioned party loyalist with an optimistic view of the future.

"I intend to make sure that next November the American people can choose between a real Republican and a real, make-no-apologies Democrat," he said.

Babbitt didn't face as many questions about his marijuana use as Gore, but was forced to rewrite his speech at the last minute because it contained several jokes about marijuana use by Judge Douglas H. Ginsburg, who withdrew today as a nominee to the Supreme Court.

Babbitt, a dark horse, attacked each of his opponents. Of Massachusetts Gov. Michael S. Dukakis, he said, "We do no service to candor when we say there's a phantom $110 billion out there to be collected by 100,000 more IRS agents. Mike, we can do better."

Earlier, three GOP candidates sparred over taxes and arms control. Rep. Jack Kemp (N.Y.) and Senate Minority Leader Robert J. Dole (Kan.) renewed their feud over taxes, with Kemp accusing Dole of leading the fight to enact "every tax increase of the 1980s."

Dole, seated next to Kemp at a televised candidate forum sponsored by the Iowa Homebuilders Association, jumped in: "A $12 billion tax increase passed in the House by one vote. And had Jack Kemp been there, it would have lost."

Kemp and former Delaware governor Pierre S. (Pete) du Pont IV reiterated their opposition to tax increases of any kind, while Dole left his options open.

"The American people are ready for bitter medicine," Dole said. "We've got to be realistic." Kemp said he believes that the country "can grow out of the deficit" and du Pont said that he would let his two rivals "argue about the present . . . . I'm going to talk about the future."

As they did in a debate in Houston last month, the three stated their reservations -- principally having to do with verifiability -- about the proposed intermediate-range nuclear forces (INF) treaty that President Reagan and Soviet leader Mikhail Gorbachev are expected to sign at a summit in Washington next month. "It's important to note that the Soviets need this summit much more than we do," Kemp said. "Their economy is a basket case. They export nothing except oppression."

Staff writer Maralee Schwartz contributed to this report.